Elementalism is the discipline of manipulating the primordial components that make up the world: Fire, Water, Earth, and Air. An elementalist can make a tidal wave of a lake, a hurricane out of still air, a burning wall from the flames of the hearth, or a protective shell from the earth below his feet. Though nothing technically stops an elementalist from using any combination of the elements, they do tend to specialize.
There is another side to Elementalism however. There exists beings known as Elementals, the physical manifestations of the elements. These beings typically manifest around particularly important or influential forms of their respective elements (an ocean or large river, a particularly large mountain, a large cerimonial pyre, or the middle of a tornado). An elementalist may harness the power of on of these beings by either making a contract with it or by forming a personal relationship and working partnership. Again, while nothing technically stops the elementalist from working with multiple different elementals, they are known to be jealous creaturs.